1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and in particular to an OCB (Optically Compensated Birefringence) mode LCD in which liquid crystal is driven at high speed.
2. Description of Related Art
LCDs with a faster response time have been in demand so as to enhance motion reproducibility of LCDs and to achieve practical application of field sequential LCDs (FS-LCDs).
A response time of an LCD refers to the time required for changing a state of liquid crystal to a drive state after a drive voltage is applied thereto. Specifically, when a voltage is applied to liquid crystal, liquid crystal molecules are aligned in a designed orientation to place the liquid crystal in a drive state. A certain amount of time is required for the alignment of liquid crystal to complete, and this time is referred to as the response time. An LCD with a slow response time has particularly poor motion display characteristics, because a prior image remains displayed for some time when displaying a motion image. Accordingly, use of an LCD employing liquid crystal with a faster response time can smooth motion image display.
In FS-LCDs, light of three primary colors are rapidly switched to alternately display images with respective colors in a single pixel, thereby achieving color display. Liquid crystal used in FS-LCDs demands a response time which is significantly faster than that used in color filter type LCDs due to the operation principle, and a practical application of FS-LCDs is highly expected.
An OCB mode liquid crystal display has been known to have a high speed response. OCB mode LCDs employ liquid crystal having a bent alignment together with a biaxial optically compensating layer. Referring to FIGS. 1A to 1C, an LCD comprises transparent substrates 51, 52 made of glass or the like and opposing each other, first and second electrodes 53, 54 and alignment films 55, 56 sequentially formed on the respective substrates, and a liquid crystal layer 57 disposed between the alignment films. The liquid crystal layer 57 comprises nematic liquid crystal. The alignment films 55, 56 are subjected to a rubbing treatment in directions substantially in parallel to each other, such that liquid crystal molecules in the liquid crystal layer 57 are oriented at a pretilt angle so as to face each other. Optically compensating layers (not shown) are disposed on the respective alignment films to achieve visible display. FIG. 1A shows an LCD when no voltage is applied to the electrodes 53, 54, in which liquid crystal molecules 57a are orientated in the rubbing direction (in the direction parallel to a document surface) while liquid crystal molecules adjacent to the alignment films 55, 56 are oriented at a pretilt angle. Referring to FIG. 1B, a drive voltage of 5V, for example, is applied to the electrode 53, and the liquid crystal molecules rise due to the applied voltage whereas molecules in the center of the liquid crystal layer 57 remain falling. A state of alignment as shown in FIG. 1B is referred to as a splay alignment. Referring to FIG. 1C, the alignment of the liquid crystal is changed to a bent alignment in which, unlike the bend alignment, the liquid crystal molecules in the center of the liquid crystal layer 57 also rise. The splay alignment and the bend alignment are reversible phase transitions. A transition of liquid crystal from a splay alignment to a bend alignment is referred to as a “bend transition”.
An OCB mode LCD is one example of an LCD using a bend alignment, in which liquid crystal with a bend alignment and a biaxial optical compensating layer are employed. Therefore, an OCB mode LCD is suitable for motion display or FS-LCDs, because liquid crystal with a bend alignment has a faster response than liquid crystal used in TN type or STN type LCDs.
There is, however, a problem that the response time significantly differs between the splay alignment before a bend transition and the bend alignment. Therefore, when producing an LCD which employs an OCB mode, a bend transition of the liquid crystal within LCD cells should be secured.
Most of the physical mechanism of a bend transition remains unknown and at present there are many problems remaining to be solved.